Prefilled syringes are used in a variety of areas, including for medical tests, therapeutic uses, and scientific uses. Prefilled syringes typically have a syringe body or syringe barrel and a plunger. The plunger includes a plunger head that seals to the inner surface of the syringe body forming a sealed cavity that holds a fluid, such as a medical test or therapeutic substance. The plunger includes a shaft coupled at one end to the plunger head, and at the other end, the shaft is coupled to a plunger top, sized to facilitate engagement by a user's finger or thumb. Pushing the plunger top forces the plunger head toward a dispensing opening located through the syringe body resulting in the dispensing or ejection of fluid or material from the syringe body.
The prefilled syringes may be used in many areas. For example, prefilled syringes may be used to hold allergen test substances. Typically, each prefilled syringe holds a volume of a single allergen test substance that may be used for multiple allergy tests for multiple patients. For example, such allergen test prefilled syringes may originally hold 5 milliliters of test substance. During an allergy test, a health care worker typically will dispense a small volume (e.g., less than 100 microliters) of test substance from the prefilled syringe into a receiving chamber that has been attached to the skin of a patient receiving an allergy test. The receiving chamber holds the allergen test substance in contact with the patient's skin, and the test area of skin is monitored for allergic reaction. Because only a small amount of the test substance is used for a single test, the prefilled syringe is used for multiple allergy tests for multiple patients. Unfortunately, it is often very difficult to accurately meter the amount of fluid or material being dispensed from the dispensing opening using the plunger of the syringe.